1. Field of the Invention
This invention relates to an optical disk on which information is recorded in pits with high density and an optical disk apparatus containing the optical disk and a playback optical system.
2. Description of the Related Art
With the recent advances in image digital signal processing techniques and moving-picture compression techniques, the latter of which have been developed by such a standardizing organization as the MPEG (Moving Picture Image Coding Experts Group), there is a growing expectation of the advent of an optical disk capable of reproducing moving-picture information such as a movie for two hours and being the same size as a CD (compact disk) in place of a VTR or laser disk. The recording capacity required to record two hours of moving-picture information in the form of analog video signals by a standard TV system such as NTSC as on the laser disk, amounts to 80 Gbyte including sound. Use of moving-picture compression techniques prescribed by a standardized method called MPEG-2, for example, requires as small a capacity as nearly 4 Gbyte even for a picture quality as good as a high picture-quality VTR such as S-VHS. The 4-Gbyte disk has been put into practical use in the form of a 300-mm diameter write-once read-many optical disk. As more and more optical disks will be used in homes in the future, it is needed to achieve an easy-to-use 120-mm diameter disk which has the same size and almost the same capacity as the CD.
The capacity of the CD format presently available as the music CD or the CD-ROM is 790 Mbyte at the maximum (when the linear velocity is 1.2 m/s). The capacity of this order can store only 24 minutes of compressed moving-picture information by MPEG-2. Thus, to store two hours of compressed moving-picture information by MPEG-2 with the CD size, the recording density must be made five times as high as that of the CD. In the current CD format, the substrate thickness is 1.2 mm, the track pitch is 1.6 .mu.m, the pit pitch is 1.66 .mu.m when the linear velocity (relative velocity between light beam and disk=disk's circumferential velocity) is 1.2 m/s, the bit length is 0.59 .mu.m, and the modulation method is EFM (eight to fourteen modulation). In the playback optical system, the playback semiconductor laser, or the laser diode (LD) has a wavelength of 780 nm, the object lens has an NA (numerical aperture) of 0.45, and the beam spot has a diameter of 1.4 .mu.m. The beam spot diameter is selected mainly from the standpoint of avoiding the effect of crosstalk between adjacent tracks.
To increase the recording density of the optical disk requires techniques for forming small pits in the disk and those for making the beam spot size small on the optical disk in the playback optical system. Concerning techniques for forming pits, for example, an optical disk matrix recording technique using Kr ion laser light (ultraviolet rays) with a wavelength of 351 nm has been proposed (The 1993 Autumn National Convention of the Applied Physics Society, 28-SF-2). This technique makes it possible to form smaller pits than a conventional Ar ion laser. In the playback optical system, by making the wavelength of the playback laser beam shorter and increasing the NA, the beam spot diameter can be made smaller. Actually, however, with conventional techniques used in CD players, even if a short wavelength light source such as a red laser diode were used, the capacity would be increased by 1.5 times at most. With such an increase in the capacity, it cannot be expected to increase the capacity by five times that of an ordinary CD, which is what is required to record two hours of compressed moving-picture information.
As described above, with the conventional optical disk techniques, to avoid the problem of crosstalk between adjacent tracks, the track pitch and pit pitch are set larger than the beam spot diameter of the playback light beam. As a result, only by making the wavelength of playback light beam shorter and increasing the NA of the object lens, the recording density cannot be raised to the extent that the capacity required to store two hours of compressed moving-picture information by MPEG2 with the CD size, for example.